1. Field of the Invention
The present invention generally relates to radar imaging systems and, more particularly, to radar target detection in environments presenting large amounts of clutter and noise to the radar receiver.
2. Description of the Prior Art
Radar is a radio device or system that detects the presence and location of an object by means of radio waves which are reflected from the object and then analyzed to determine characteristics of the object. Since the invention of radar for location of distant objects, numerous techniques of signal processing have been used to enhance the image obtained from the returned signal and to reduce noise. Recent coherent radar systems have improved the imaging of the object to obtain additional, high-resolution signatures of the object for purposes of identification. However, system noise and clutter remain a major obstacle to radar imaging.
In seagoing applications, in particular, waves of the water surface are a major source of clutter which may obscure the object to be detected. Detection of an object is achieved when the returned signal has been processed to the point that an object present, amid clutter and noise, can be differentiated from the false detections caused by environmental artifacts, particularly time-varying clutter such as ocean waves.
Modern coherent radar systems detect objects by storing a transmitted signal or waveform which is then combined with the returned signal in a synchronous or phase sensitive detector to produce in-phase (I) and quadrature (Q) samples at baseband. These samples are then processed and compared to a threshold to determine detection. With ideal coherent detection, the phase of the signal from the target must be known, a priori. However, the obtaining of information concerning the location and velocity of a target, both of which affect phase, is the basic objective of a radar system. Ideal coherent detection therefore serves as a basis for comparison for other types of radar systems rather than being a practical or even realizable radar system.
In the general case where the target is in motion relative to the detector, the phase of the target signal is not known and coherent detection cannot easily be used. For example, a phase history for each of a large plurality of possible signal trajectories could be synthesized in a similarly large plurality of coherent detectors and the results compared to determine detection. Accordingly, non-coherent so-called envelope detectors in which the returned signals are analyzed using a square-law device (e.g. I.sup.2 +Q.sup.2) are more commonly used.
Envelope detectors do not require a knowledge of phase but this absence of information results in a loss of sensitivity compared with coherent detectors. This loss of sensitivity implies that a higher signal-to-noise ratio (SNR) is required for similar levels of performance, generally specified as a receiver operating characteristic (ROC). Receiver operating characteristics are plots of the probability of detection against the probability of false alarm for different signal-to-noise ratios.
More recently, higher-order statistics (HOS) have been used by the inventor to analyze the output of a coherent radar synchronous detector in the frequency domain as disclosed in U.S. Pat. Nos. 5,227,801 and 5,231,403, to Robert D. Pierce which are hereby fully incorporated by reference, in order to obtain down-range profiles and to obtain an estimation of velocity of moving targets, respectively. HOS methods have the advantages of preserving phase information, insensitivity to linear phase shifts and suppression of Gaussian noise. However, these techniques have assumed detection of the target prior to performing these functions and operate in the domain of the radar's carrier frequency wherein the output is stepped in frequency to obtain down-range resolution.
Another application of higher-order statistics to noise reduction during signal detection in an imaging system is disclosed in "Signal Detection and Classification Using Matched Filtering and Higher Order Statistics" by Georgios B. Giannakis and Michael K. Tsatsanis, published in IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. 38, No. 7, July 1990. The method of signal detection disclosed therein is based on using the higher-order statistics related to signal energy as a detection test statistic. An estimate of the zeroth lag of the kth-order correlation is computed. The third-order correlation is zero for the complex I and Q (e.g. I+iQ) samples but the fourth-order correlation can be used as a detector. For each set of samples in this technique, the test statistic is developed from fourth-order and second order averages of the I and Q samples taken at each time step. The second-order average accumulates the I and Q magnitude squared which is the same as the quadratic detector test statistic. The fourth-order average accumulates the I and Q magnitude raised to the fourth power and contains second order terms related to the additive Gaussian noise for which estimates are made and removed. This involves doubling the averages of the accumulated second-order products and subtracting them from the average of the accumulated fourth order products.
Since the technique disclosed by Giannakis et al. involves correcting an estimate with another estimate, uncertainty of detection is increased. Also, the removal of second-order estimates requires a significant fraction of the processing time of the technique. Further, the technique of Giannakis et al. is, in the article, applied to image extraction in which noise may be more likely to be accurately described by Gaussian distributions. The spiky and modulated nature of time-varying sea clutter is expected to have deleterious effects on the performance of the fourth-order detector of Giannakis et al.